Iodine Prophylaxis and Health Concern

5.1 How is radiation related to thyroid cancer?

Radioactive iodine can be released during nuclear emergencies. If breathed in or swallowed, it will concentrate in the thyroid gland and increase the risk of thyroid cancer. The risk of thyroid cancer following radiation exposure is higher in children and young adults. Among persons exposed to radioactive iodine, the risk of thyroid cancer can be lowered by taking potassium iodide pills, which helps prevent the uptake of the radioactive iodine.

5.2 How much iodine do we need from our diet? What is the consequence of excessive intake?

World Health Organization (WHO) recommended a requirement of 150 μg/day iodine for adults. It is generally considered that intake of not exceeding 1000 μg iodine from diet daily would not cause adverse health effect to average adults. Moreover, the WHO recommended the upper limit (UL) of iodine for adults should not exceed 30 μg/kg bw/day. For example, for adults weighing 60 kg, iodine intake should not exceed 1800 μg/day (i.e. 1.8 mg/day). The UL is the highest nutrient intake value that is likely to pose no risk of adverse health effects for individuals in a given age and gender group.

Excessive intake of iodine can cause thyroid disorders, including hypothyroidism and hyperthyroidism.

5.3 Why is there a need to take very high level of iodine as prophylaxis in radioactive incident? Is it suitable for everyone as a general preventive measure against radiation contamination?

In the setting of a nuclear power plant accident, potassium iodide pills are given to saturate the thyroid gland and prevent the uptake of radioactive iodine. When given before or shortly after exposure, this step can reduce the risk of cancer in the long term.

However, potassium iodide pills are not "radiation antidotes". They do not protect against external radiation, or against any other radioactive substances besides radioactive iodine. They may also cause medical complications for some individuals such as persons with poorly functioning kidney (please see related questions by the Department of Health at http://www.dh.gov.hk/faq_eng.html).

5.4 What is the function of iodine in human body?

Iodine is an essential nutrient required for normal bodily function. It is necessary for the synthesis of the thyroid hormones which play a key role in regulating various metabolic functions in the human body. Iodine deficiency may lead to goitre, hypothyroidism, growth and mental retardation in children, etc.

5.5 What are the main sources of iodine in our diet?

Diet is the major source of iodine intake for humans. Certain foods such as marine fish and seaweeds are naturally relatively rich in iodine. Iodine is also present in many other foods, including iodized salt.

5.6 Why some salt in the market is being iodized?

Salt is commonly considered as a vehicle for iodine fortification for the general population. In some counties (especially developing counties), the population intake of iodine is relatively low. Salt iodization is mandatory or recommended in these countries to prevent the risk of iodine deficiency of the population.

5.7 Can iodine-rich foods (including seaweed) and iodized salt be taken as a prophylactic measure in radiation emergencies?

Iodine-rich foods (including seaweeds) and iodized salt are not appropriate to be used for prophylactic purpose in radiation emergencies. For example, one has to consume over 200 grams of seaweeds a day in order to obtain the equivalent dose of iodine that is contained in an iodine pills (i.e. 100 mg of iodine per pill). Excessive intake of iodine-rich foods for a long time may lead to thyroid disorders (please see the related question above). For iodized salt, its iodine content is relatively low. Moreover, one has to take about 2.5 to 5 kg of iodized salt a day in order to absorb the equivalent dose of iodine that is contained in an iodide tablet. Consuming an excessive amount of salt is harmful to health especially for those with high blood pressure, heart disease or kidney disease.

5.8 Is there any international standard or recommendation on salt iodization?

The World Health Organization (WHO) has recommended a level of 20-40 mg/kg for iodisation of salt.

5.9 What are the adverse effects of consuming too much salt?

Many studies revealed that excessive intake of salt could lead to hypertension and increased risk of cardiovascular disease. In order to prevent chronic diseases, WHO recommended that salt intake should not exceed 5 gram per person per day.

Ingestion of 500-1000mg salt per kg body weight (i.e., intake of 6-12 teaspoon salt for an adult weighed 60kg) can be toxic to human, with symptoms including gastrointestinal tract ulceration, muscle dysfunction, and renal damage. It would also lead to dehydration and affect the nervous system. However, it is rare to have excessive intake of salt that would lead to toxicity responses.